Electron beam apparatuses, in particular a scanning electron microscope (also referred to as SEM below) and/or a transmission electron microscope (also referred to as TEM below), are used to examine objects (also referred to as samples) in order to obtain knowledge in respect of the properties and behavior of the objects under certain conditions.
In an SEM, an electron beam (also referred to as primary electron beam below) is generated by means of a beam generator and focused on an object to be examined by way of a beam guiding system. An objective lens is used for focusing purposes, and said objective lens will be discussed in greater detail further below. The primary electron beam is guided in a raster manner over the surface of the object to be examined by way of a deflection device. Here, the electrons of the primary electron beam interact with the object to be examined. Interaction particles and/or interaction radiation is/are produced as a result of the interaction. By way of example, the interaction particles are electrons. In particular, electrons are emitted by the object—the so-called secondary electrons—and electrons of the primary electron beam are scattered back—the so-called backscattered electrons. The secondary electrons and backscattered electrons are detected by means of at least one particle detector. The particle detector generates detection signals, which are used to generate an image of the object. Thus, an image of the object to be examined is obtained. By way of example, the interaction radiation comprises x-ray radiation and/or cathodoluminescence radiation. The interaction radiation is detected by means of at least one radiation detector, which generates detection signals. By way of example, these detection signals are used to generate spectra, by means of which properties of the object to be examined are determined.
In a TEM, a primary electron beam is likewise generated by means of a beam generator and focused on an object to be examined by means of a beam guiding system. The primary electron beam passes through the object to be examined. When the primary electron beam passes through the object to be examined, the electrons of the primary electron beam interact with the material of the object to be examined. The electrons passing through the object to be examined are imaged onto a luminescent screen or onto a detector—for example in the form of a camera—by a system having an objective. By way of example, the aforementioned system additionally also comprises a projection lens. Here, imaging can also take place in the scanning mode of a TEM. Such a TEM is generally referred to as STEM. Additionally, provision can be made for detecting electrons scattered back at the object to be examined and/or secondary electrons emitted by the object to be examined by means of a further detector in order to image an object to be examined.
The integration of the function of an STEM and an SEM in a single particle beam apparatus is known. It is therefore possible to carry out examinations of objects with an SEM function and/or with an STEM function using this particle beam apparatus.
Furthermore, the prior art has disclosed the practice of analyzing and/or processing an object in a particle beam apparatus using, on the one hand, electrons and, on the other hand, ions. By way of example, an electron beam column having the function of an SEM is arranged at the particle beam apparatus. Additionally, an ion beam column is arranged at the particle beam apparatus. Ions used for processing an object are generated by means of an ion beam generator arranged in the ion beam column. By way of example, material of the object is ablated or material is applied onto the object during the processing. The ions are additionally or alternatively used for imaging. The electron beam column with the SEM function serves, in particular, for examining the object, but also for processing the object.
In all of the above-described embodiments of a particle beam apparatus, in general use is made of an objective lens comprising pole pieces in which one coil is arranged or a plurality of coils are arranged. In particular, it is known to provide an objective lens with a first coil and with a second coil. The first coil and the second coil are wound for example onto an individual coil carrier. To put it more precisely, a first coil wire of the first coil and a second coil wire of the second coil are wound onto the individual coil carrier. For example, the first coil wire and the second coil wire are wound on one another. As an alternative thereto, provision is made for guiding the first coil wire and the second coil wire in a bifilar fashion. This means that the first coil wire and the second coil wire are wound closely alongside one another and parallel to one another around the individual coil carrier. The individual coil carrier may have a cavity through which a cooling liquid flows in order to achieve a cooling of the first coil and of the second coil.
The first coil and the second coil are supplied with a respective coil current by different current sources. This means that a first current source provides a first coil current for the first coil. Furthermore, a second current source provides a second coil current for the second coil. In order to control the first coil current to a specific value, a first current control unit is provided in the known particle beam apparatuses. Furthermore, a second current control unit is provided in order to control the second coil current to a specific value. In the known particle beam apparatuses provision is made for always controlling separately the first coil current, on the one hand, and the second coil current, on the other hand. In order to determine the individual currents, that is to say the first coil current and the second coil current, measuring resistors are provided in the current control devices. To put it more precisely, provision is made of a first measuring resistor for determining the first coil current and a second measuring resistor for determining the second coil current. A first through-flow current is determined by determining a voltage drop across the first measuring resistor. A second through-flow current is determined by determining a voltage drop across the second measuring resistor. It may happen here that a drift of the resistance value of the first measuring resistor and of the second measuring resistor may occur on account of heating of the first measuring resistor and of the second measuring resistor. Said drift may be different for the first measuring resistor and the second measuring resistor. This leads to different measurement errors when determining the individual through-flow currents and ultimately to erroneous control of the first coil current and/or of the second coil current.
Furthermore, provision is made for performing a so-called power-constant driving of the objective lens in the known particle beam apparatuses. This means that the power that is generated by the objective lens is intended always to be constant. To put it another way, the following is intended to hold true: P=I12·R1+I22·R2=K, wherein P is the power of the objective lens, I1 is the first coil current, I2 is the second coil current, R1 is a first resistance, R2 is a second resistance, and K is a constant. The abovementioned resistances are the resistances of the first coil and of the second coil. As an alternative thereto, they may also be the resistances of the measuring resistors used. For simplification it is assumed that the coils of the objective lens behave like ohmic resistors. As a result of the driving with a constant power, however, in the first coil and in the second coil an increase in the temperature occurs which is not compensated for by the abovementioned cooling with the cooling liquid. The temperature is accordingly not reduced again. The increase in the temperature of the first coil and/or of the second coil brings about a thermal expansion of the first coil, of the second coil and/or of the pole pieces. This thermal expansion may result in mechanical alterations of the first coil, of the second coil and/or of the pole pieces in such a way that fields generated by the first coil, the second coil and/or the pole pieces change. This may influence the focusing of the particle beam by the objective lens. In particular, inaccuracies in the focusing of the particle beam may occur.
The circuit arrangements for controlling the coil currents as known heretofore from the prior art have relatively high noise, which may lead to inaccuracies in the control of the coil currents.
Accordingly, it is desirable to be able to provide a particle beam apparatus and a method for operating a particle beam apparatus with which low-noise control of the current for the coils of an objective lens is possible, such that the focusing of a particle beam is influenced as little as possible.